1. Field of the Invention
This invention relates to a sol-gel process for processing gel monoliths for use in fabricating oxide-based glasses and ceramics with controlled properties (e.g., chemical purity, homogeneity) compatible with a variety of applications, including high-performance optics.
2. Description of the Related Art
High-performance oxide-based materials are increasingly in demand for use in a variety of applications. For example, silica glass has the optical transmittance, mechanical hardness, chemical durability, thermal stability, low thermal expansion, and high laser damage threshold which make it an optimal material for applications such as optoelectronic laser diodes, fiber optic telecommunications, medical laser delivery systems, and military optical sensors. There is significant pressure on material manufacturers to find fabrication techniques which can satisfy the increasingly stringent performance requirements for silica glass and other oxide-based materials.
Numerous techniques are currently in use for the fabrication of oxide-based glasses or ceramics. For example, silica glasses have traditionally been manufactured by melting natural quartz or synthetic silica in crucibles at high temperatures (typically about 1700xc2x0 C.-2000xc2x0 C.). However, the resultant materials have limited utility for various optical applications, primarily due to structural inhomogeneities as well as impurity concentrations (e.g., from intrinsic impurities in the raw materials, incomplete chemical reactions of components, and contamination by the crucible). Such high-temperature processes are also incompatible with manufacturing products with certain compositions, tailored dopant or additive gradients, organic or high vapor pressure additives, or additives in their metallic or partially reduced states.
Another more recent technique for manufacturing silica glasses has been chemical vapor deposition (CVD), in which silicon-containing chemical vapors are combined with oxygen under high temperature conditions to deposit silica onto a substrate. However, the resultant materials are relatively expensive due to low material collection efficiencies, slow processing rates, and complex processing and pollution control equipment. Furthermore, CVD processes lack the versatility for fabricating more compositionally complex glasses.
Sol-gel technology has been identified as possibly being able to fabricate products which satisfy some or all of the desired performance requirements without the difficulties or limitations found in more conventional fabrication techniques. A typical sol-gel silica process involves the transition of a liquid colloidal solution xe2x80x9csolxe2x80x9d phase into a solid porous xe2x80x9cgelxe2x80x9d phase, followed by drying and sintering the resulting gel monolith at elevated temperatures to form silica glass. One method of preparing a silica porous gel monolith is to pour into a mold a solution of silica-forming compounds (e.g., silicon alkoxides and tetraethyl orthosilicate (TEOS)), solvents, and catalysts, which then undergoes hydrolysis and polymerization, resulting in a wet porous gel monolith or matrix. After drying the wet gel monolith in a controlled environment to remove the fluid from the pores, the dry gel monolith is densified into a solid glass-phase monolith.
Sol-gel technology can yield products with the desired chemical purity, homogeneity, and flexibility in compositions, dopants, and dopant profiles. However, the potential for sol-gel processes for fabricating large monoliths has been limited by various problems. Large gel monoliths can take a long time to dry, thereby limiting the product throughput. But even more importantly, shrinkage of the gel monolith during the drying process often results in cracking, especially in larger gel monoliths.
The present invention provides a method of using sol-gel technology to process oxide-based monoliths.
One aspect of the present invention is a method of processing an oxide-based gel monolith having pores filled with liquid, an inner region, and an outer region. The method comprises drying the gel monolith such that the outer region is not dried before the inner region.
Another aspect of the present invention is a method for processing an oxide-based gel monolith comprising pores filled with liquid, an inner region, and an outer region. The method comprises substantially drying the gel monolith by drawing liquid from a surface of the gel monolith at a rate greater than the pore liquid diffusion rate and then decreasing the rate of drawing liquid from the surface to less than or approximately equal to the pore liquid diffusion rate, such that the outer region of the gel monolith is not dried before the inner region of the gel monolith is dried. The substantially dried gel monolith is sintered, whereby the gel monolith is further densified and is transformed into a glass or ceramic phase material.
Another aspect of the present invention is a method for processing an oxide-based gel monolith of an initial volume, wherein the gel monolith comprises pores filled with liquid. The method comprises removing liquid from the gel monolith at a first relatively rapid rate until the volume of the gel monolith reaches a predetermined first percentage of the initial volume, and subsequently removing liquid from the gel monolith at a second rate, substantially lower than the first rate, until the gel monolith reaches a second percentage of the initial volume less than said first percentage of the initial volume.
Another aspect of the present invention is a method for processing an oxide-based wet porous gel monolith having an inner region including the center of the gel monolith, and an outer region including the surface of the gel monolith. The method comprises substantially drying the gel monolith by exposing the gel monolith to conditions in which white, opaque features are exhibited in the inner region of the gel monolith before white, opaque features are exhibited in the outer region of the gel monolith.
Another aspect of the present invention is a method for processing an oxide-based gel monolith comprising pores filled with liquid. The method comprises placing the gel monolith in a drying oven, and subjecting the gel monolith to a drying treatment comprising heating said gel monolith in the drying oven without controlling the humidity of the atmosphere within the drying oven.
Yet another aspect of the present invention is a method for processing an oxide-based gel monolith comprising pores filled with liquid. The method comprises placing the gel monolith in a drying oven, and subjecting the gel monolith to a drying treatment comprising only heating said gel monolith in the drying oven.